Drop ejection devices are used for a variety of purposes, most commonly for printing images on various media. They are often referred to as ink jets or ink jet printers. Drop-on-demand drop ejection devices are used in many applications because of their flexibility and economy. Drop-on-demand devices eject one or more drops in response to a specific signal, usually an electrical waveform, or waveform, that may include a single pulse or multiple pulses. Different portions of a multi-pulse waveform can be selectively activated to produce the drops. One or more drive pulses build a drop from a nozzle of the drop ejection device.
Drop ejection devices typically include a fluid path from a fluid supply to a nozzle path. The nozzle path terminates in a nozzle opening from which drops are ejected. Drop ejection is controlled by pressurizing fluid in the fluid path with an actuator, which may be, for example, a piezoelectric deflector, a thermal bubble jet generator, or an electrostatically deflected element. A typical printhead has an array of fluid paths with corresponding nozzle openings and associated actuators, and drop ejection from each nozzle opening can be independently controlled. In a drop-on-demand printhead, each actuator is fired to selectively eject a drop at a specific target pixel location as the printhead and a substrate are moved relative to one another.
Drop ejection devices need to generate drops sustainably, obtain a required drop volume, deliver material accurately, and achieve a desired delivery rate. Drop placement errors with respect to a target degrade image quality on the target. FIG. 1 illustrates different types of drop placement errors. A drop 120 is fired through a nozzle plate 110 towards a target 130. Vertical line 170 represents an ideal straight drop trajectory. However, a nozzle error 180 results from a misalignment of the nozzle with respect to the target. Vertical line 180 represents a straight drop trajectory from the nozzle to the target with this line being orthogonal to the nozzle plate 110. An angle theta formed between the vertical line 180 and the actual trajectory 190 of the drop represents the jet trajectory error 150. A total drop placement error equals the combination of nozzle placement error and jet trajectory error.
A “permanent” jet straightness occurs when a jet is always straight or always crooked. Jets that are permanently crooked are generally a result of nozzle damage and/or contamination in or around the nozzle. Transient jet straightness occurs when a jet that is straight immediately after priming goes crooked after a period of jetting. These jets may or may not self-recover after a further period of jetting. A jet trajectory error arises from crooked jets. FIGS. 2 and 3 illustrate examples of crooked jets. Area 202 illustrates jets that are crooked in the same direction. Area 204 illustrates twinning in which adjacent jets are crooked in opposite directions. FIG. 3 illustrates the printed areas that result from crooked jets. Arrow 210 points to an area in which crooked jets cause the line-to-line distance to become uneven. Arrow 220 points to an area in which transient jet straightness causes the position of printed lines to change over a period of time. Arrow 230 points to an area in which twinning causes two neighboring lines to merge into one line. In either case, the image quality produced from the crooked jets is degraded.